The primary goal of these studies is an understanding of the regulation of tyrosine hydroxylase in the central nervous system. We have previously shown an activation of the soluble enzyme by phosphorylation or catecholamine removal. Tyrosine hydroxylase activity is now being studied in a synaptosomal system, which provides conditions more similar to those found in vivo. Synaptosomes prepared from mouse whole brain, rat striatum and rat cortex have been used to study the regulation of tyrosine hydroxylase activity in synaptosomes from mouse whole brain and rat cortex, but not rat striatum. In addition, we have found that tyrosine hydroxylase activity in whole brain and cortical synaptosomes is affected by extrasynaptosomal tyrosine concentration, and that addition of calcium changes the kinetics of tyrosine hydroxylase for tyrosine. Tyrosine hydroxylase activity in striatal synaptosomes did not show this tyrosine dependency.